Why Humid Heat Is Dangerous
Your body cools itself by sweating. Sweat carries heat away as it evaporates, and evaporation depends on how much water the surrounding air can still absorb. When humidity is high the air is already close to saturated, sweat evaporates slowly, and the cooling mechanism stalls.
That is what the heat index expresses: the temperature that dry air would need to reach to stress your body as much as the current combination of heat and humidity does. At 90°F with 20% humidity you are comfortable enough; at 90°F with 80% humidity you are in danger.
The Formula
The National Weather Service uses a polynomial fitted by Lans Rothfusz in 1990 to a much more complex biometeorological model:
HI = −42.379 + 2.04901523T + 10.14333127R − 0.22475541TR − 0.00683783T2
− 0.05481717R2 + 0.00122874T2R + 0.00085282TR2 − 0.00000199T2R2
with T in Fahrenheit and R as relative humidity in percent. Two adjustments apply at the edges: the value is reduced in very dry heat below 13% humidity, and increased in very humid conditions above 85%. Below about 80°F a simpler averaging form is used instead.
The model assumes a person of average build in the shade, walking at 3.1 mph, wearing long trousers and a short-sleeved shirt. Direct sunlight can add as much as 15°F to the real feel.
Heat Index Chart
Apparent temperature in °F:
| Humidity | 80°F | 85°F | 90°F | 95°F | 100°F | 105°F | 110°F |
|---|---|---|---|---|---|---|---|
| 40% | 80 | 85 | 91 | 99 | 109 | 121 | 135 |
| 50% | 81 | 88 | 96 | 107 | 120 | 135 | 150 |
| 60% | 82 | 91 | 100 | 114 | 132 | 149 | — |
| 70% | 83 | 93 | 106 | 124 | 144 | — | — |
| 80% | 84 | 97 | 113 | 136 | — | — | — |
| 90% | 86 | 102 | 122 | — | — | — | — |
| 100% | 87 | 108 | 132 | — | — | — | — |
Risk Categories
| Heat index | Category | What can happen |
|---|---|---|
| Below 80°F | Safe | No heat stress expected |
| 80–90°F | Caution | Fatigue with prolonged exposure or activity |
| 91–103°F | Extreme caution | Heat cramps and heat exhaustion possible |
| 104–125°F | Danger | Heat exhaustion likely, heat stroke possible |
| 126°F and above | Extreme danger | Heat stroke highly likely |
Recognising Heat Illness
| Condition | Signs | What to do |
|---|---|---|
| Heat cramps | Painful muscle spasms, heavy sweating | Rest in shade, drink fluids with electrolytes |
| Heat exhaustion | Heavy sweating, cool clammy skin, weakness, nausea, headache | Move to a cool place, loosen clothing, sip water, cool with damp cloths |
| Heat stroke | Body temperature above 40°C, hot dry or damp skin, confusion, loss of consciousness | Medical emergency. Call for help immediately and cool aggressively |
The critical distinction: in heat exhaustion the body is still sweating and fighting; in heat stroke thermoregulation has failed. Confusion or altered behaviour in the heat should always be treated as an emergency.
Wet-Bulb Temperature
A related and increasingly discussed measure is the wet-bulb temperature — the lowest temperature achievable by evaporative cooling alone. A wet-bulb reading of 35°C (95°F) is the theoretical survival limit: at that point a healthy person cannot shed heat at all, even resting in shade with unlimited water, and will overheat within hours. Recent research has recorded brief excursions above that threshold in parts of South Asia and the Persian Gulf.
Who Is Most at Risk
- Older adults — reduced sweating capacity and blunted thirst response.
- Young children — higher surface-area-to-mass ratio and less efficient sweating.
- Outdoor workers and athletes — sustained exertion in direct sun.
- People on certain medications — diuretics, beta blockers, antihistamines and some antipsychotics impair heat regulation.
- Those with heart or lung conditions, and anyone not acclimatised to the heat. Acclimatisation takes 10–14 days of gradual exposure.
Frequently Asked Questions
Why does the calculator refuse below 70°F?
The equation was fitted to hot conditions and is not meaningful in cool weather. For cold conditions use the wind chill calculator, which models heat loss rather than heat retention.
Is the heat index the same as humidex?
No. Humidex, used in Canada, is derived from dew point rather than relative humidity and produces different numbers. Both aim at the same idea and neither is directly comparable to the other.
Does shade matter?
Greatly. The index assumes shade; full sun can add up to 15°F to the apparent temperature.
Does a fan help in extreme heat?
Only up to a point. Once air temperature exceeds skin temperature, around 35°C, a fan blows hot air over you and can increase heat gain. Above that, cooling or wetting the skin is what works.